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There's no limit, although you should make sure that all images will be seen within the period of your ad exposure with enough appreciation time.
What is the background image URL for?
The background image URL is an image you assign to a certain story to serve as back-up visuals for live campaigns.
What specs do I use for the background image URL?
You may assign any size for the image as long as it is high resolution and doesn't pixelize.
In what file can the Ad Formats be exported to?